In wavelength division multiplexing (WDM) optical communication systems a single optical fiber may be used to carry multiple optical signals. In a single-carrier-based WDM system, each optical signal is generated by directly modulating a high symbol rate (e.g. 32 GBd) baseband data stream on an associated optical channel. Each optical channel may be at an associated wavelength that is separated from adjacent channels by a defined channel-spacing, e.g. according to a channel plan established by the International Telecommunications Union. The optical channels are then multiplexed to form an aggregate or WDM signal.
Single-carrier-based WDM systems have dominated commercial system implementations for long-haul, e.g. transoceanic, optical transmission. In an effort to improve transmission performance, subcarrier multiplex (SCM) systems have been investigated. In a SCM system a high symbol rate baseband data stream may be broken down into multiple low symbol rate (e.g. several GBd) baseband data streams. The low symbol rate baseband data streams may each be modulated by a different electrical subcarrier frequency and then multiplexed to form a high symbol rate subcarrier aggregate signal. The subcarrier aggregate signal is modulated by an optical carrier and transmitted as an optical signal. As illustrated for example in FIG. 1, in the transmitted optical signal each of the lower symbol rate baseband data streams occupies a different portion p1, p2 . . . p10 of the optical spectrum surrounding the center wavelength λ0 of the optical carrier. SCM systems may be implemented using WDM by modulating multiple different subcarrier aggregate signals on different associated optical channels and multiplexing the optical channels for transmission on an optical transmission path.
Transmitted optical signals propagate on the optical transmission path to a receiver. The receiver detects and demodulates the data. Unfortunately, one problem associated with any optical communication system is maintaining the integrity of the data being communicated, particularly when optical signals are transmitted over long distances in long-haul communication systems. Accumulated contributions of several different linear and/or non-linear impairments associated with the transmission path may cause degradation of the signals and may cause difficulty in differentiating between the binary digits (i.e., the ones and zeros) in a data stream at the receiver.